Opportunistic pathogens such as Candida species can use carboxylic acids, like acetate and lactate, to survive and successfully thrive in different environmental niches. These nonfermentable substrates are frequently the major carbon sources present in certain human body sites, and their efficient uptake by regulated plasma membrane transporters plays a critical role in such nutrient-limited conditions. Here, we cover the physiology and regulation of these proteins and their potential role in Candida virulence. This review also presents an evolutionary analysis of orthologues of the Saccharomyces cerevisiae Jen1 lactate and Ady2 acetate transporters, including a phylogenetic analysis of 101 putative carboxylate transporters in twelve medically relevant Candida species. These proteins are assigned to distinct clades according to their amino acid sequence homology and represent the major carboxylic acid uptake systems in yeast. While Jen transporters belong to the sialate:H+ symporter (SHS) family, the Ady2 homologue members are assigned to the acetate uptake transporter (AceTr) family. Here, we reclassify the later members as ATO (acetate transporter ortholog). The new nomenclature will facilitate the study of these transporters, as well as the analysis of their relevance for Candida pathogenesis.

Candida albicans is an opportunistic fungal pathogen that is highly resistant to different oxidative stresses. How reactive sulfur species (RSS) such as sulfite regulate gene expression and the role of the transcription factor Zcf2 and the sulfite exporter Ssu1 in such responses are not known. Here, we show that C. albicans specifically adapts to sulfite stress and that Zcf2 is required for that response as well as induction of genes predicted to remove sulfite from cells and to increase the intracellular amount of a subset of nitrogen metabolites. Analysis of mutants in the sulfate assimilation pathway show that sulfite conversion to sulfide accounts for part of sulfite toxicity and that Zcf2-dependent expression of the SSU1 sulfite exporter is induced by both sulfite and sulfide. Mutations in the SSU1 promoter that selectively inhibit induction by the reactive nitrogen species (RNS) nitrite, a previously reported activator of SSU1, support a model for C. albicans in which Cta4-dependent RNS induction and Zcf2-dependent RSS induction are mediated by parallel pathways, different from S. cerevisiae in which the transcription factor Fzf1 mediates responses to both RNS and RSS. Lastly, we found that endogenous sulfite production leads to an increase in resistance to exogenously added sulfite. These results demonstrate that C. albicans has a unique response to sulfite that differs from the general oxidative stress response, and that adaptation to internal and external sulfite is largely mediated by one transcription factor and one effector gene.

Candida albicans shares communal niches with multiple bacterial species. Previous work from our group demonstrated that the Gram-positive bacterium Enterococcus faecalis, a normal constituent of the oral and gut microbiome that is often co-isolated with C. albicans, antagonizes hyphal morphogenesis, biofilm formation, and virulence in C. albicans. These effects are mediated by EntV, a bacteriocin and antimicrobial peptide produced by E. faecalis. The main aim of this work is to unveil the molecular mechanism behind the activity of EntV on C. albicans. Using fluorescence microscopy, we determined that EntV binds to the cell walls of several Candida species, including both yeast and hyphae of C. albicans. Contrary to other antimicrobial peptides, it does not cause cell lysis and does not synergize with cell wall damaging agents. Moreover, we screened a library of C. albicans mutants for strains with altered susceptibility to the peptide; most of the positive hits had functions related to cell wall maintenance and were further screened to ascertain changes in the staining patterns. Furthermore, to identify the target layer on the cell wall, pull-down assays were performed. Mannan was identified as the major wall component able to bind the peptide. Finally, live imaging of macrophages incubated with Candida was carried out in order to assess any change in the phagocytic behaviour in presence of the peptide. Identifying the molecular target of EntV in regard to the anti-virulence mechanisms of C. albicans is an important step in its further development as a therapeutic addition to the classical antifungal agents.

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